Several chemical composition measurement devices using light spectrometers are currently commercially available. Examples of the types of spectrometers currently used include Fourier transform infrared spectrometer (FTIR), dispersive spectrometer (spectrograph or monochromator) and linear variable filter (LVF) spectrometer. FTIR based devices use Michelson interferometers and have generally been considered to provide the highest performance, due to their high optical throughput, which enables high-sensitivity measurements. In contrast, dispersive and linear variable filter spectrometers have significantly lower optical throughput and thus lower sensitivity performance. However, dispersive and linear variable filter spectrometers generally provide simpler and more rugged instrumentation, and are less expensive to manufacture.
Another type of chemical composition measurement and monitoring device that is widely used, in particular, in the field of gas monitoring, is non-dispersive infrared (NDIR) devices. These devices use fixed narrowband optical filters to select a particular wavelength band region. They have high optical throughput, rivaling that of FTIR based devices, and thus provide high-sensitivity measurement. This type of device, however, is generally not considered to be a spectrometer, as it does not measure light intensity as a function of wavelength; rather, it provides a single measurement value corresponding to the light intensity at a particular wavelength band. For this reason, each device (employing one filter, one photo-detector and one light source) can only measure one compound. Therefore, such devices are not considered to be chemical “composition” measuring devices.
The transmitted wavelength band of a narrowband optical filter, such as that used in NDIR instruments, can be varied or “tuned” by varying the angle of incidence (U.S. Pat. No. 4,040,747 to Webster, 1977 and U.S. Pat. No. 2,834,246 to Foskett, 1958, both of which are incorporated herein by reference). Such methods enable the measurement of optical signals from multiple wavelengths or wavelength bands using only a single optical filter, light source and detector, thus potentially creating a simple, low-cost, high-throughput spectrometer.
One method of varying the incident angle is to continuously rotate the filter in one direction and capture the data at the relevant angular positions. This type of continuously-rotating filter spectrometers has been described (U.S. Pat. No. 4,040,747 to Webster, 1977, U.S. Pat. No. 2,834,246 to Foskett, 1958, U.S. Pat. No. 5,268,745 to Goody, 1993, U.S. Pat. No. 7,099,003 to Saptari, 2006, all of which are incorporated herein by reference). However, these devices have not been in significant commercial use. FTIR spectrometers, grating based spectrometers and LVF spectrometers are still by far the most commonly used hardware for chemical composition monitoring, despite the potential advantages for the rotating tunable filter instruments.
There are weaknesses of the previous rotating tunable filter systems which prevent them from being used in a commercial setting as chemical composition measuring or monitoring devices. For example, these systems lack measurement stability and robustness due to wavelength instability, spectral interferences, environmental variations and/or instrumental changes. Such systems also lack versatility, in particular, in that they are not able to provide wide spectral coverage. Furthermore, there are difficulties in volume manufacturing, in particular, difficulties in producing reproducible instruments that are interchangeable without each instrument requiring empirical sample based calibration.